In ultrasound imaging and more particularly in medical echography, the choice of frequency is dictated by the compromise between resolution and penetration depth. Specifically, because of the increase in attenuation of ultrasound waves with frequency, the penetration depth of ultrasound increases with decreasing frequency. However, the image resolution decreases with decreasing frequency.
In addition, a process for the investigation and display of human tissues is known, through document U.S. Pat. No. 5,178,148, for determining the volume of a tumour or of a gland using signals coming from a probe steered by the process.
Processes are known, in particular through patent FR 2,620,327, for the investigation of ocular structures, by echography, using probes operating at low frequencies of the order of 10 MHz, and focused to a depth roughly equal to the size of an ocular globe (about 23 to 25 mm). These processes mean, on one hand, that images in section of the posterior segment of the eye can be achieved with spatial resolutions of the order of a millimeter and, on the other hand, that a very rough examination of the entire anterior segment of the eye can be carried out.
The major drawback of low-frequency echography is mainly the low resolution (600 to 700 μm) provided by these low frequencies, which do not allow detailed analysis of the retina and the other layers of the posterior wall of the eye (choroid and sclera) and more particularly in the macular region.
In order to increase both the lateral and axial resolution, investigation and display processes using ultrasound probes at high frequency, of the order of 50 to 100 MHz (cf. U.S. Pat. No. 5,551,432 and C. J. PAVLIN, M. D. SHERAR, F. S. FOSTER: “Subsurface ultrasound microscopic imaging of the intact eye”, Ophthalmology 97: 244, 1990), with a short focal length (of about 4 to 8 mm), have enabled the use, with a resolution of 50 μm, of structures of the anterior segment of the eye, to depths of the order of 5 mm, or of structures of the peripheral retina which are very close to the anterior segment.
In conclusion, it is therefore accepted that the use of high frequencies seems to be limited to investigation of the anterior segment and the peripheral retina, whereas investigation of the deep structures (posterior segment) requires the use of much lower frequencies, while only providing very low spatial resolutions, of a few hundred microns.